Phase noise is noise that manifests itself as unwanted, usually random, fluctuations in a relative phase of a signal. Many modern systems, such as communications systems that employ one or more forms of phase modulation, phased array radars and the global positioning satellite system (GPS), depend on precise, accurate knowledge of signal phase for their operation. Phase noise directly interferes with the operation of these and related systems. Therefore, the measurement of phase noise is an important topic in the field of test and measurement. In particular, an important objective of phase noise measurement is to obtain an accurate measurement of a true phase noise for a given signal under test (SUT).
There are a number of approaches known in the art for measuring phase noise, each method having its own advantages and disadvantages. For example, phase noise can be measured using a frequency discriminator or using a delay line and mixer as a frequency comparator. Another approach uses two sources and a phase comparator. Yet another approach measures phase noise using a heterodyne frequency measurement technique. However, among all of the approaches to phase noise measurement, perhaps the most popular and practical approach uses a spectrum analyzer.
A spectrum analyzer is a device or system that measures the power spectral density (PSD) of a signal or one of several closely related signal parameters. For the purposes of discussion herein, the PSD of a signal can be thought of as a measurement of signal power in a selected bandwidth as a function of frequency. Typically, the spectrum analyzer displays measured PSD in the form of a graph. Phase noise measurements can be extracted or computed from measured PSD of the SUT.
Most spectrum analyzers, especially those used for high frequency RF and microwave signals, are implemented as heterodyne receivers that frequency shift or frequency convert the signal prior to detecting and measuring the power of the signal. A typical spectrum analyzer may employ three or four stages of frequency conversion prior to a signal power measurement. One or more of these frequency conversion stages generally utilizes a swept or stepped frequency local oscillator (LO) to provide frequency scanning or tuning.
Unfortunately, phase noise measurements obtained using spectrum analyzers inevitably contain errors that distort and in some cases, even obscure the true phase noise of the signal under test. In practice, the LOs used in the various frequency conversions along with aspects of detecting and measuring signal power in the spectrum analyzer introduce or add phase noise to the SUT being measured. The added phase noise is typically independent of the signal being measured and is solely due to the operations performed by the spectrum analyzer on the SUT. For example, a major source of added phase noise in the spectrum analyzer is phase noise of a first LO used in a first frequency conversion stage of the spectrum analyzer. The end result is that the magnitude of the phase noise, as measured by the spectrum analyzer, is generally greater than the true or actual phase noise of the SUT due to this added phase noise.
A conventional approach to mitigating the added phase noise effects of the spectrum analyzer used to measure phase noise of the SUT normally involves simply using a better spectrum analyzer. In simple terms, a better spectrum analyzer is one that has lower added phase noise. The lower the added phase noise, the less that added phase noise corrupts the measurements of phase noise of the SUT.
Lower added phase noise in a spectrum analyzer is typically achieved by using cleaner, more stable LOs. This is especially true for the LO used in the first frequency conversion stage or stages. In addition, added phase noise of a spectrum analyzer can often be reduced by increasing the resolution of the signal detection circuitry of the spectrum analyzer. Thus, a better spectrum analyzer, having lower added phase noise, is the result of using better, higher quality components to construct the spectrum analyzer.
However, improving the added phase noise performance in a spectrum analyzer (i.e. reducing added phase noise) typically comes at a price. Even moderate improvements in phase noise performance from one model of spectrum analyzer to another can often result in significant increases in unit price. The increased unit price is due in large part to increased costs of better, higher performance LOs and/or higher resolution detection circuitry necessary to implement spectrum analyzers with lower added phase noise. Much the same thing can be said for the other phase noise measurement approaches known in the art. Increased phase noise measurement accuracy using better measurement devices can become very, sometimes even prohibitively, expensive.
Accordingly, it would be nice to have a more economical approach to obtaining accurate phase noise measurements than simply using a better, more expensive spectrum analyzer. Moreover, it would be advantageous if such an approach could improve accuracy of phase noise measurements produced by virtually any spectrum analyzer, even ones with lower added phase noise. Such an approach would solve a long-standing need in the area of economical phase noise measurement using spectrum analyzers.